The present invention relates to semiconductor devices, and more particularly to a method and system for qualifying an ONO layer in a semiconductor device.
FIG. 1 depicts a portion of a conventional semiconductor device 10, such as a Flash memory device. The conventional semiconductor 10 utilizing an oxide-nitride-oxide (xe2x80x9cONOxe2x80x9d) layer 13 formed on a semiconductor substrate 12. The ONO layer 13 includes two oxide layers separated by a nitride layer 16. The first oxide layer, which is closest to the substrate 12 is a tunnel oxide layer 14. The upper oxide layer is a control oxide layer 18. The thinned portion of the control oxide layer 18 corresponds to a bitline 20 that runs perpendicular to the plane of FIG. 1. The nitride layer 14 acts as a charge storage layer, or a floating gate. Thus, charges can tunnel through the tunnel oxide layer 14 and be trapped on the nitride layer 14. As a result, the threshold voltage of a device utilizing the ONO layer 13 is changed. In order to alter the threshold voltage, a voltage is typically applied to the control oxide layer 18.
Typically, devices made using the ONO layer 13 are desired to be qualified, or investigated to determine that their properties meet certain specifications. In particular, it is desirable to ensure that when the ONO layer 13 is included in a device, such as a flash memory device, the ONO layer 13 will have a particular lifetime. It is, therefore, desirable to qualify the ONO layer 13.
One property of the ONO layer 13 desired to be determined during qualification is the lifetime of the ONO layer 13. Predicting the lifetime of the ONO layer 13, particularly in structures such as the bitline 20, is difficult. The ONO layer 13 is a multiplayer. As such, different layers within the ONO layer, such as the tunnel oxide 14, the nitride 16 and the control oxide 18, may have different properties. These layers 14, 16 and 18 within the ONO layer 13 therefore have different lifetimes. As a result, the lifetime of the ONO layer 13 could vary. Consequently, reliably predicting the lifetime of the ONO layer 13 is difficult.
Accordingly, what is needed is a system and method for qualifying an ONO layer. The present invention addresses such a need.
A method and system for qualifying an oxide-nitride-oxide (ONO) layer including a first oxide layer, a nitride layer and a control oxide layer in a semiconductor device is disclosed. The method and system comprise determining a first plurality of lifetimes for the ONO layer. The first plurality of lifetimes is determined utilizing a first plurality of applied voltages at a first plurality of temperatures for each of a first plurality of ONO layers having a particular nitride layer thickness and a plurality of control oxide layer thicknesses. The method and system also comprise determining a second plurality of lifetimes for the ONO layer. The second plurality of lifetimes are determined utilizing a second plurality of applied voltages at a second plurality of temperatures for each of a second plurality of ONO layers having a particular control oxide layer thickness and a plurality of nitride layer thicknesses. The method and system also comprise determining a field acceleration factor for the ONO layer based on the first plurality of lifetimes, the second plurality of lifetimes, the first plurality of applied voltages and the second plurality of applied voltages. In addition, the method and system comprise determining an activation energy for the ONO layer based on the first plurality of lifetimes, the second plurality of lifetimes, the first plurality of temperatures and the second plurality of temperatures. The method and system also comprise determining a lifetime for the ONO layer based upon the field acceleration factor and the activation energy for the ONO layer.
According to the system and method disclosed herein, the present invention provides a method for qualifying an ONO layer, as well as selecting thicknesses of the control oxide and nitride layer that can improve performance of the ONO layer.